Level Adjustment And Headphones Amplifier - Revox B225 Maintenance Instructions Manual

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B225
SECTION 3/8*
The preemphasis is corrected with the circuit illustratec
in Fig. 3.13. The subcode on the CD specifies whether
the selection was recorded with or without preemphasis.
The characteristic of the corresponding filter can
be adjusted with the PREEM signal.
3.6.2 Level adjustment and headphones amplifier
The volume is controlled by a DUAL D/A converter
(AD7528). It serves as an attenuator before the
corresponding input (IC103a, pin 2 = left-hand channel
/ IC 203b
f
pin 6 = right-hand channel). The negative-
feedback resistances of both opamps remain unchanged
which means that the level can only be attenuated with
this circuit. The dual D/A converter is controlled by a
CMOS shift register through the signals DLEN-3, DATA, and
CLK. The circuit illustrated in Fig. 3.14 constitutes the
output amplifier for the headphones socket and also
the VARIABLE OUTPUT* The signal is also attenuated for
this purpose through a voltage divider so that the signal
is the same (2 V RMS) at both outputs (FIXED and
VARIABLE). The signal is taken through relay Kl in order
to prevent power-on switching clicks. When power is
applied., the relay is selected by the
KILL signal (which rises to +5 V after approximately
1 to 2 seconds). As a result, the outputs are only
through-connected when the supply voltages are stable.